The process of the present invention relates to a "post-press" or second press operation for creating thick, hard, integral surface portions on at least one and preferably both surfaces of a man-made fiberboard or particle board substrate. A handleable mat is first manufactured by consolidating a cellulosic fiber-containing composition to form a consolidated fiberboard or particle board. The handleable mat can be consolidated by drying a mat formed by depositing a water slurry of fibers onto a water pervious support member, as is common in the manufacture of insulation board, or the mat can be consolidated under heat and pressure until substantially dry in a first press operation. This consolidated mat is thereafter "post-pressed" to form a surface skin to improve the stiffness, strength, paint holdout and design fidelity properties of the board. In accordance with one embodiment disclosed in our prior U.S. Pat. No. 4,175,148 a low density core material of less than 35 pounds/ft..sup.3 is maintained and a skin, formed in the after consolidation press step with the skin forming chemical, is a higher density surface portion. In accordance with a second embodiment disclosed herein, heavier skinned surface products are formed in the same manner but starting with higher density consolidated mats on the order of 35 pounds/ft..sup.3 or greater, for example, 35-65 pounds/ft..sup.3. For the purpose of the present invention, a "post-press" operation refers to a press step performed on the mat after consolidation thereof.
The fiberboard of the present invention is generally formed in two separate operations. The first operation forms a condolidated mat while the second or "post-press" operation creates the skinned surface portions. It should be noted however, that consolidated man-made boards previously manufactured are readily available and can be used as the consolidated mat to impart a skin to such consolidated mats in accordance with the present invention. It is quite surprising that a "post-press" or second pressing operation on a consolidated mat surface treated with a particular skin forming chemical is effective in forming a smooth surface skin on the mat. Prior to the present invention, it was thought that the qualities and physical characteristics of a board would be finally determined by the conditions under which the board is initially consolidated and by the raw materials used to form the board, because of the bonding occuring during consolidation.
Much of the bonding necessary for cohesion and strength in a man-made board occurs during the consolidation of the board during hot-pressing. Prior to drying a board made by the wet process, the board is weak, but the bonding forces created during the final consolidation are generally quite powerful. Generally the same bonding effect occurs in the final consolidation of a board made by the dry process. Prior to hot-pressing, the loosely disassociated cellulosic fibrous product is quite weak, but after hot-pressing the mat into its final configuration, it is very powerfully held together by the bonding which occurs during hot-pressing. Hot-pressing during consolidation causes a welding or coalescing of the cellulose fibers at the surface of the product so that the surface portion consists of wood remade in modified form. It is very difficult to densify or restructure the surface of a consolidated product without again destroying these fiber-to-fiber surface welds. It is especially difficult to densify only a surface portion of a consolidated board in an amount sufficient to substantially increase the strength and stiffness of the product while retaining the lightweight core material to provide a lightweight product. This same concept can be used to improve and restructure the surface of higher density core materials by post-pressing consolidated mats having a density of 35 pounds/ft..sup.3 or greater. The process described herein accomplishes these results.
A prior patent, assigned to Masonite, U.S. Pat. No. 4,007,076, discloses and claims a method of post-press embossing consolidated, man-made boards to effectively and accurately reshape the surface of a completed panel without surface defects such as surface fiber tearing. In accordance with U.S. Pat. No. 4,007,076, a previously consolidated panel is post-press embossed at a pressure of at least 1000 psi to create a surface design, or to permanently redesign the surface of a consolidated product without destroying the interfiber bonding established at the surface of the board during consolidation. The product thus formed does not include a skin formed by post-pressing with a skin forming chemical and does not disclose retaining a relatively low density core or central portion in accordance with one embodiment of the present invention.
In accordance with the present invention, it has been found that the surface physical characteristics of a consolidated mat can be altered in a post-press operation to harden, smooth, and densify the surfaces thereof when at least the surface fibers of the mat are contacted with a skin forming chemical. During the post-press operation, at a temperature of at least 525.degree. F., the skin forming chemical reacts to stiffen and strengthen the cellulosic substrate, such as insulation board and fiberboard, to provide the necessary strength for the intended purpose of the substrate while, in one embodiment, retaining a relatively low density, lightweight central core of the material and in another embodiment, using a consolidated mat of at least 35 pounds/ft..sup.3 density, improving the stiffness, strength and surface properties of paint holdout and design fidelity.